SCR on Low Thermal Mass Filter Substrates

ABSTRACT

Provided are selective catalytic reduction (SCR) filters that effectively provide simultaneous treatment of particulate matter and NO x . Provided also are methods for reducing NO x  concentration and particulate matter in a diesel engine exhaust by using the SCR filters. The SCR filter can include a fiber matrix wall flow filter comprising a plurality of non-woven inorganic fibers and a chabazite molecular sieve SCR catalyst on the fiber matrix wall flow filter. By combining a fiber matrix wall flow filter with a chabazite molecular sieve SCR catalyst, high catalyst loading can be achieved without causing excessive back pressure across the filter when implemented in emission treatment systems.

RELATED APPLICATIONS

This application is a divisional of U.S. Utility patent application Ser.No. 12/036,019, filed on Feb. 22, 2008, the disclosure of which ishereby incorporated by reference. This application also claims priorityto U.S. Provisional Patent Application 60/891,835 filed on Feb. 27,2007, entitled COPPER CHA ZEOLITE CATALYSTS, which is herebyincorporated by reference.

BACKGROUND

Over many years harmful components of nitrogen oxides (NO_(x)) havecaused atmospheric pollution. NO_(x) is contained in exhausted gasessuch as from internal combustion engines (e.g., automobiles and trucks),from combustion installations (e.g., power stations heated by naturalgas, oil, or coal), and from nitric acid production plants.

Different methods have been used in the treatment of NO_(x)-containinggas mixtures. One type of treatment involves catalytic reduction ofnitrogen oxides. There can be two processes: (1) a nonselectivereduction process wherein carbon monoxide, hydrogen, or a lowerhydrocarbon is used as a reducing agent, and (2) a selective reductionprocess wherein ammonia or ammonia precursor is used as a reducingagent. In the selective reduction process, a high degree of removal withnitrogen oxide can be obtained with a small amount of reducing agent.

The selective reduction process is referred to as SCR process (SelectiveCatalytic Reduction). The SCR process uses catalytic reduction ofnitrogen oxides with ammonia in the presence of atmospheric oxygen withthe formation predominantly of nitrogen and steam:

-   -   4NO+4NH₃+O₂→4N₂+6H₂O    -   2NO₂+4NH₃+O₂→3N₂+6H₂O    -   NO+NO₂+NH₃→2N₂+3H₂O

A diesel engine exhaust contains phase materials (liquids and solids)which constitute particulates or particulate matter as well as NO_(x).Often, catalyst compositions and substrates on which the compositionsare disposed are provided in diesel engine exhaust systems to convertcertain or all of these exhaust components to innocuous components. Forexample, diesel exhaust systems can contain one or more of a dieseloxidation catalyst, a soot filter, and a catalyst for the reduction ofNO_(x).

The particulate matter emissions of diesel exhaust contain three maincomponents. One component is a solid, dry, solid carbonaceous fractionor soot fraction. This dry carbonaceous matter contributes to thevisible soot emissions commonly associated with diesel exhaust. A secondcomponent of the particulate matter is a soluble organic fraction(“SOF”). The soluble organic fraction is sometimes referred to as avolatile organic fraction (“VOF”). The VOF can exist in diesel exhausteither as a vapor or as an aerosol (fine droplets of liquid condensate)depending on the temperature of the diesel exhaust. The VOF arises fromtwo sources: (1) lubricating oil swept from cylinder walls of the engineeach time the pistons go up and down; and (2) unburned or partiallyburned diesel fuel. A third component of the particulate matter is asulfate fraction. The sulfate fraction is formed from small quantitiesof sulfur components present in the diesel fuel. Small proportions ofSO₃ are formed during combustion of the diesel, which in turn combinesrapidly with water in the exhaust to form sulfuric acid. The sulfuricacid collects as a condensed phase with the particulates as an aerosol,or is adsorbed onto the other particulate components, and thereby addsto the mass of total particulate matter.

SUMMARY

The following presents a simplified summary of the innovation in orderto provide a basic understanding of some aspects described herein. Thissummary is not an extensive overview of the claimed subject matter. Itis intended to neither identify key or critical elements of the claimedsubject matter nor delineate the scope of the subject innovation. Itssole purpose is to present some concepts of the claimed subject matterin a simplified form as a prelude to the more detailed description thatis presented later.

The subject innovation described herein relates to selective catalyticreduction (SCR) filters that effectively provide simultaneous treatmentof particulate matter and NO_(x). The subject innovation also relates toemission treatment systems and emission treatment methods that involvethe SCR filter. The SCR filter can include a fiber matrix wall flowfilter comprising a plurality of non-woven inorganic fibers and achabazite molecular sieve SCR catalyst on the fiber matrix wall flowfilter. By combining a fiber matrix wall flow filter with a chabazitemolecular sieve SCR catalyst, which is an SCR catalyst with thechabazite structure, a high catalyst loading can be achieved withoutcausing excessive back pressure across the filter when implemented inemission treatment systems.

The subject innovation also relates to methods for reducing NO_(x)concentration and particulate matter in a diesel engine exhaust. Inaccordance with one aspect of the claimed subject matter, the methodinvolves injecting ammonia or an ammonia precursor a diesel engineexhaust, and passing the exhaust through a SCR filter containing a fibermatrix wall flow filter and a chabazite molecular sieve SCR catalyst onthe fiber matrix wall flow filter, the fiber matrix wall flow filtercontaining a plurality of non-woven inorganic fibers.

The following description and the annexed drawings set forth in detailcertain illustrative aspects of the claimed subject matter. Theseaspects are indicative, however, of but a few of the various ways inwhich the principles of the innovation may be employed and the claimedsubject matter is intended to include all such aspects and theirequivalents. Other advantages and novel features of the claimed subjectmatter will become apparent from the following detailed description ofthe innovation when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a fiber matrix wall flow filter inaccordance with one aspect of the specification.

FIG. 1B is a cutaway view of a portion of a fiber matrix wall flowfilter in accordance with one aspect of the specification.

FIGS. 2A and 2B are schematic diagrams illustrating systems for treatingan exhaust stream containing NO_(x) and particulate matter in accordancewith one aspect of the specification.

FIG. 3 is a flow diagram of an exemplary methodology for reducing NO_(x)concentration and particulate matter in a diesel engine exhaust inaccordance with one aspect of the specification.

DETAILED DESCRIPTION

The subject innovation described herein relates to selective catalyticreduction (SCR) filters, emission treatment systems, emission treatmentmethods that effectively provide simultaneous treatment of particulatematter and NO_(x). Integration of NO_(x) reduction and particulateremoval functions into a single catalyst article can be accomplished byusing a fiber matrix wall flow filter coated with a chabazite molecularsieve SCR catalyst. The emission treatment system uses an integratedsoot filter and SCR catalyst to significantly minimize the weight andvolume required for the emissions system. Moreover, due to the choice ofcatalytic compositions implemented in the system, effective pollutantabatement can be provided for exhaust streams of varying temperatures.This feature is advantageous for operating diesel engines under varyingloads and engine speeds which have a significant impact on exhausttemperatures emitted from the engines.

The subject innovation can be used in an application where highfiltration efficiency is required. For example, the SCR filter issuitable for effectively removing particulate matter in emissiontreatment systems. The combination of a fiber matrix wall flow filterand a chabazite molecular sieve SCR catalyst disclosed herein allowswall flow substrates to be loaded with practical levels of the SCRcatalyst without causing excessive back pressure across the coatedfilter when implemented in emission treatment systems.

Compared to conventional emission treatment techniques, improvedflexibility in emission treatment system configuration and lower fuelcosts associated with active regeneration of the system can be achievedby a combination of a fiber matrix wall flow filter having low thermalmass and a hydrophobic chabazite molecular sieve as a SCR catalyst. Bycoating the filter with the chabazite molecular sieve SCR catalyst, thelower thermal mass of the filter can provide faster light-off, betterconversions at low temperature for NO_(x) control by the SCR reaction,and high temperature stability.

The filter contains fused fiber bundles to form a wall flow depthfilter. This structure generates high porosity while achieving goodfiltration efficiency. Even after coating the SCR catalyst on the filterwall, there is substantially no negative interaction of the coating withthe substrate to adversely affect the physical properties. The matrixstructure with the high loading of SCR coating indicates only a minimalincrease in back pressure over the uncoated substrate.

NO_(x) is reduced with ammonia (NH₃) to nitrogen (N₂) over the SCRcatalyst. Urea is typically used to form ammonia. The filter can provideefficient mixing of urea with the catalyst. By improving the mixing ofthe urea and the SCR catalyst within the fiber matrix body, hydrolysisof the urea proceeds faster, thereby enabling NH₃ to be made morereadily for reaction with the NO_(x).

The fiber matrix wall flow filter can contain a plurality of non-woveninorganic fibers. The non-woven inorganic fibers can be any suitablefiber as long as the fibers can have thermal tolerance under emissiontreatment processes. The non-woven inorganic fibers can have one or moreproperties of a high melting point, low heat conductance, lowcoefficient of thermal expansion, ability to withstand thermal andvibrational shock, low density, and high porosity and permeability.Thus, the fiber matrix wall flow filter containing the non-woveninorganic fibers can have one or more properties of a high meltingpoint, low heat conductance, low coefficient of thermal expansion, anability to withstand thermal and vibrational shock, a low density, ahigh porosity, and a high permeability.

General examples of non-woven inorganic fibers include alumina fibers,silica fibers, mullite fibers, silicon carbide fibers, aluminosilicatefibers, aluminum borosilicate fibers, or the like. The alumina fiberstypically contain about 95 wt. % or more and about 97 wt. % or less ofalumina and about 3 wt. % or more and about 5 wt. % or less of silica ina fibrous form. The alumina fibers can be produced by extruding orspinning a solution of precursor species.

The silica fibers typically contain about 90 wt. % or more of amorphoussilica with low impurity levels. In one embodiment, silica fibers has alow density (e.g., about 2.1 g/cm³ or more and about 2.2 g/cm³ or less),high refractoriness (about 1600 degrees Celsius), low thermalconductivity (about 0.1 W/m-K), and near zero thermal expansion.

The aluminosilicate fibers typically contain about 40 wt. % or more andabout 80 wt. % or less of alumina, about 5 wt. % or more and about 25wt. % or less of silica, and about 0 wt. % or more and about 20 wt. % ofiron or magnesium oxides

The aluminum borosilicate fibers typically contain about 40 wt. % ormore and about 80 wt. % or less of alumina, about 5 wt. % or more andabout 25 wt. % or less of silica, and about 1 wt. % or more and about 30wt. % of boric oxide or boron oxide. The details of the aluminumborosilicate fibers can be found in, for example, U.S. Pat. No.3,795,524, which is hereby incorporated by reference.

The fibers can have any suitable average fiber diameter for forming themonolithic honeycomb structure of the fiber matrix wall flow filter. Inone embodiment, the fibers have about 0.5 microns or more and about 50microns or less of average fiber diameter. In another embodiment, thefibers have about 0.7 microns or more and about 30 microns or less ofaverage fiber diameter. In yet another embodiment, the fibers have about1 micron or more and about 20 microns or less of average fiber diameter.

The fibers can have any suitable average tensile strength for formingthe monolithic honeycomb structure of the fiber matrix wall flow filter.In one embodiment, the fibers have an average tensile strength greaterthan about 700 MPa (100,000 psi). In another embodiment, the fibers havean average tensile strength greater than about 1,200 MPa (200,000 psi).In yet another embodiment, the fibers have an average tensile strengthgreater than about 1,800 MPa (300,000 psi). In still yet anotherembodiment, the fibers have an average tensile strength greater thanabout 2,100 MPa (350,000 psi).

The fiber matrix wall flow filter can contain alumina fibers, silicafibers, mullite fibers, silicon carbide fibers, aluminosilicate fibers,aluminum borosilicate fibers, or the like at suitable weight percentage.In one embodiment, the inorganic fiber portion of the filter containsfrom about 50 wt. % or more to about 90 wt. % or less of silica fibers,from about 5 wt. % or more to about 50 wt. % or less of alumina fibers,and from about 10 wt. % or more to about 25 wt. % or less of aluminumborosilicate fibers.

In one embodiment, the fiber matrix wall flow filter further containsadditives such as binding agents and thickening agents. Organic bindersand inorganic binders such as boron binders (e.g., boron nitride) can beadded. Alternatively, boron nitride can be added to replace aluminumborosilicate fibers. For example, the filter contains silica fiber,alumina fiber, and boron nitride in similar weight percentage asdescribed above.

In one embodiment, the filter contains low density fused fibrous ceramiccomposites prepared from amorphous silica and/or alumina fibers withabout 2 to about 12% boron nitride fibers by weight. The details of thelow density fused fibrous ceramic composites can be found in, forexample, U.S. Pat. No. 5,629,186, which is hereby incorporated byreference.

The monolithic honeycomb structure of the fiber matrix wall flow filtercan be formed by any suitable technique. In one embodiment, the filteris formed by forming a blank via sol-gel techniques and forming holes orcells via drilling in the blank. In another embodiment, the filter isformed by extrusion techniques. The details of the manufacture offilters are not critical to the subject innovation. The details of themanufacture of filters can be found in, for example, U.S. PatentApplication Publication No. 2004/0079060, which is hereby incorporatedby reference.

The fiber matrix wall flow filter can have a relatively low thermalmass, which in turn can contribute to faster heating and shorterlight-off times. Since the fiber matrix wall flow filter can be quicklyheated to the catalyst activation temperature, catalysts on the filtercan quickly begin to convert NO_(x) in the exhaust gas into N₂.

The fiber matrix wall flow filter can have a low coefficient of thermalexpansion between about 200 degrees Celsius and about 800 degreesCelsius (CTE 200-800). In one embodiment, the filter with or without acoating of a SCR catalyst has a CTE 200-800 of about 1×10⁻⁶ /degreeCelsius or more and about 6×10⁻⁶/degree Celsius or less. In anotherembodiment, the filter with or without a coating of a SCR catalyst has aCTE 200-800 of about 2×10⁻⁶/degree Celsius or more and about4.5×10⁻⁶/degree Celsius or less. In yet another embodiment, the filterwith or without a coating of a SCR catalyst has a CTE 200-800 of about3×10⁻⁶/degree Celsius or more and about 4×10⁻⁶/degree Celsius or less.

The fiber matrix wall flow filter can also have a low coefficient ofthermal expansion between about 900 degrees Celsius and about 500degrees Celsius (CTE 900-500). In one embodiment, the filter with orwithout a coating of a SCR catalyst has a CTE 900-500 of about 200 ppmor more and about 1500 ppm or less. In another embodiment, the filterwith or without a coating of a SCR catalyst has a CTE 900-500 of about300 ppm or more and about 1000 ppm or less. In yet another embodiment,the filter with or without a coating of a SCR catalyst has a CTE 900-500of about 350 ppm or more and about 500 ppm or less.

The fiber matrix wall flow filter can have an elastic or Young'smodulus, Emod. The Emod of the wall flow filter can be measured at roomtemperature or at elevated temperature from 200 to 1000° C., forexample. In one embodiment, the room temperature Emod values can rangefrom about 0.9 to about 1.2 Mpsi for an uncoated fiber wall flow filtermaterial. In another embodiment, the fiber filter material can have aroom temperature Emod value of about 0.8 to about 1.4 when coated.

The fiber matrix wall flow filter can have a modulus of rupture strength(MOR). In one embodiment, the filter with or without a coating of a SCRcatalyst has a MOR of about 1,000 psi or more and about 2,000 psi orless when measured at room temperature in a typical four point bendingtest in a manner similar to ASTM C 1161-02c. In another embodiment, thefilter with or without a coating of a SCR catalyst has a MOR of about1,000 psi or more and about 1,800 psi or less. In yet anotherembodiment, the filter with or without a coating of a SCR catalyst has aMOR of about 1,000 psi or more and about 1,500 psi or less.

The fiber matrix wall flow filter can have a thin porous walledhoneycomb structure through which a fluid stream passes without causinga great increase in back pressure or pressure across the filter. Thefilter can have any suitable honeycomb cell density. In one embodiment,the honeycomb cell density is about 100 cell/in² or more and about 400cell/in² or less. In another embodiment, the honeycomb cell density ofthe filter is about 200 cell/in² or more and about 300 cell/in² or less.The honeycomb cell shape can be square, triangle, round, oval,pentagonal, Hepa, doughnut, or the like. The inlet channel can be largerthan outlets to reduce backpressure generation and ash storage capacity.The wall thickness of the honeycomb structure can be about 10 mils ormore and about 40 mils or less. In another embodiment, the wallthickness of the honeycomb structure can be about 20 mils or more andabout 30 mils or less. The porosity of the wall of the honeycombstructure can be about 60% or more and about 90% or less. In anotherembodiment, the porosity of the wall of the honeycomb structure is about70% or more and about 85% or less. In yet another embodiment, theporosity of the wall of the honeycomb structure is about 55% or more andabout 70% or less. The pore size can be about 15 microns or more andabout 100 microns or less. In one embodiment, the pore size can be about15 microns or more and about 30 microns or less.

Any fiber matrix wall flow filter having the above mentioned propertiescan be suitable for use in the practices of the subject innovation.Specific examples of such fiber matrix wall flow filter can be found in,for example, U.S. Patent Application Publication Nos. 2004/0079060,2005/0042151, 2006/0120937, 2007/0104621, 2007/0104622, 2007/0104620,2007/0151799, 2007/0151233, 2007/0107395, 2007/0152364, 2007/0111878,2007/0141255, 2007/0107396, 2007/0110645, 2007/0108647, 2007/0220871,2007/0207070, and 2007/0104632, which are hereby incorporated byreference.

A molecular sieve can be zeolitic—zeolites—or non-zeolitic, and zeoliticand non-zeolitic molecular sieves can have the chabazite crystalstructure, which is also referred to as the CHA structure by theInternational Zeolite Association. Zeolitic chabazite include anaturally occurring tectosilicate mineral of a zeolite group withapproximate formula: (Ca,Na₂,K₂,Mg)Al₂Si₄O₁₂.6H₂O (e.g., hydratedcalcium aluminum silicate). Three synthetic forms of zeolitic chabaziteare described in “Zeolite Molecular Sieves,” by D. W. Breck, publishedin 1973 by John Wiley & Sons, which is hereby incorporated by reference.The three synthetic forms reported by Breck are Zeolite K-G, describedin J. Chem. Soc., p. 2822 (1956), Barrer et al; Zeolite D, described inBritish Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat.No. 3,030,181, which are hereby incorporated by reference.

Synthesis of another synthetic form of zeolitic chabazite, SSZ-13, isdescribed in U.S. Pat. No. 4,544,538, which is hereby incorporated byreference, by using N-alkyl-3-quinuclidinol,N,N,N-tri-alkyl-1-adamantylammonium cations and/orN,N,N-trialkyl-exoaminonorbornane as a directing agent in a conventionalOH-medium. SSZ-13 typically contains a silica to alumina molar with aratio of about 8 to about 50. The molar ratios can be adjusted byvarying the relative ratios of the reactants in the synthesis mixtureand/or by treating the zeolite with chelating agents or acids to removealuminum from the zeolite lattice. The crystallization of SSZ-13 can beaccelerated and the formation of undesirable contaminants can be reducedby adding seeds of SSZ-13 to the synthesis mixture.

Synthesis of a synthetic form of a non-zeolitic molecular sieve havingthe chabazite crystal structure, silicoaluminophosphate 34 (SAPO-34) andSAPO-37, is described in U.S. Pat. No. 7,264,789, which is herebyincorporated by reference, by using a colloidal suspension of seeds tocrystallize the chabazite structures. A method of making yet anothersynthetic no-zeolitic molecular sieve having chabazite structure,SAPO-44, is described in U.S. Pat. No. 6,162,415, which is herebyincorporated by reference. These zeolitic and nonzeolitic molecularsieves having the chabazite structure, SSZ-13, SAPO-34, and SAPO-44 canbe employed as a SCR catalyst in the subject innovation.

Chabazite molecular sieves can be hydrophobic. Hydrophobic chabazitemolecular sieve means that the chabazite molecular sieve is hydrophobicin and of itself, or that the chabazite molecular sieve is ahydrophilicchabazite molecular sieve that is rendered hydrophobic byapplication of an outer coating of a suitable hydrophobic wetting agent(e.g., the particulate material has a hydrophilic core and a hydrophobicouter surface).

The surfaces of chabazite molecular sieves can be made hydrophobic bycontact with hydrophobic wetting agents. Any suitable mineralapplications, especially in organic systems such as plastic composites,films, organic coatings, or rubbers, can be employed to render thechabazite molecular sieve surface hydrophobic. The details of themineral applications are described in, for example, Jesse Edenbaum,Plastics Additives and Modifiers Handbook, Van Nostrand Reinhold, NewYork, 1992, pages 497-500, which is hereby incorporated by reference forteachings of surface treatment materials and their application.

General examples of surface treatment materials include coupling agentssuch as fatty acids and silanes. Specific examples of hydrophobic agentsinclude: organic titanates such as Tilcom® obtained from TioxideChemicals; organic zirconate or aluminate coupling agents obtained fromKenrich Petrochemical, Inc.; organofunctional silanes such as Silquest®products obtained from Witco or Prosil® products obtained from PCR;modified silicone fluids such as the DM-Fluids obtained from Shin Etsu;and fatty acids such as Hystrene® or Industrene® products obtained fromWitco Corporation or Emersol® products obtained from Henkel Corporation.In one embodiment, fatty acids and salts thereof (e.g., stearic acid andstearate salts) are employed to render a particle surface of chabazitemolecular sieves hydrophobic.

The hydrophobicity refers to the physical property of a surface of thechabazite molecular sieve to dislike or repel water. Hydrophobicity canbe described by using contact angle measurements. The contact angle isdefined by equilibrium forces that occur when a liquid sessile drop isplaced on a smooth surface. The tangent to the surface of the convexliquid drop at the point of contact among the three phases, solid (S),liquid (L), and vapor (V) is the contact angle θ.

The relationship between the surface tension of the solid-vapor (γ sv),liquid-vapor (γ_(LV)), and solid-liquid (γ_(SL)) can be defined by thefollowing Young's equation:

F=γρcos θ

where F=wetting force; γ=liquid surface tension; and ρ=wettingperimeter.

If the water droplet spreads out on the surface, the contact angle isless than 90 degrees and the surface is hydrophilic. If the surface ishydrophobic then the contact angle is greater than 90 degrees. Thus, 180degrees is the maximum hydrophobicity that a surface can have.

Many surfaces change their surface energy upon contact with water.Dynamic contact angle measurements provide both an advancing andreceding contact angles. The advancing contact angle is a measurement ofthe surface hydrophobicity upon initial contact with a liquid, while thereceding contact angle measures the hydrophobicity after the surface hasbeen wetted with a liquid. Thus, in one embodiment, for the purposes ofthe subject innovation, “hydrophobic” or “hydrophobicity,” when used inreference to chabazite molecular sieve, chabazite molecular sieveparticles have an advancing and/or receding contact angle of about 90degrees or more. In another embodiment, chabazite molecular sieveparticles have an advancing and/or receding contact angle of about 100degrees or more. In yet another embodiment, chabazite molecular sieveparticles have an advancing and/or receding contact angle of about 110degrees or more. In still yet another embodiment, chabazite molecularsieve particles have an advancing and/or receding contact angle of about120 degrees or more.

In one embodiment, chabazite molecular sieve particles have a recedingcontact angle of about 90 degrees or more. In another embodiment,chabazite molecular sieve particles have a receding contact angle ofabout 100 degrees or more. In yet another embodiment, chabazitemolecular sieve particles have a receding contact angle of about 110degrees or more. In still yet another embodiment, chabazite molecularsieve particles have a receding contact angle of about 120 degrees ormore.

The dynamic contact angles are based on a gravimetric principle of theWilhelmy plate technique and are determined by measurement on a DynamicContact Angle Instrument which can measure both advancing and recedingcontact angles of powdered samples. A dynamic contact angle analysissystem (model DCA 315) from ATI Cahn Instruments Inc. can be used forcontact angle measurements. The surface tension (γ) of deionized wateris determined with a standard platinum calibration plate. Powder samplesare deposited on dual sided adhesive tape. The perimeter (ρ) of the tapeis determined with a caliper. The impregnated tape is placed in the DCA315 and lowered and raised in the deionized water at a rate of 159microns/second for two immersion cycles. The contact angles can bedetermined from the advancing and receding wetting hysteresis curves ofthe first immersion cycle.

Chabazite molecular sieves in the subject innovation can be ionexchanged chabazite molecular sieves. Cations of the ion exchangedchabazite molecular sieve can be any suitable metal cation. Examples ofmetal cations include a transition metal selected from the groupconsisting of copper, chromium, iron, cobalt, nickel, iridium, cadmium,silver, gold, platinum, manganese, and mixtures thereof. Resulting ionexchanged chabazite molecular sieve can be Cu-exchanged chabazitemolecular sieve, Fe-exchanged chabazite molecular sieve, or the like.

The degree of desired ion exchange is not narrowly critical. Chabazitemolecular sieves can be exchanged with such a cation to a point at whichthe exchanged cation represents any suitable ratio of exchanged ions toAl cations. In one embodiment, ion-exchanged chabazite molecular sievecontains about 0.3 exchanged ions to Al atomic ratio or more. In anotherembodiment, ion-exchanged chabazite molecular sieve contains about 0.6exchanged ions to Al atomic ratio or more. In yet another embodiment,ion-exchanged chabazite molecular sieve contains about 0.7 exchangedions to Al atomic ratio or more. In still yet another embodiment,ion-exchanged chabazite molecular sieve contains about 0.8 exchangedions to Al atomic ratio or more.

The ion-exchanged chabazite molecular sieve can be formed by exchangingcations of a precursor chabazite molecular sieve with other cations.Exchanging cations can be achieved in any suitable technique such asimmersion techniques. A precursor chabazite molecular sieve can beimmersed into a solution containing soluble salts of metal species. ThepH of the solution can be adjusted by addition of ammonium hydroxide, toinduce precipitation of the metal cations onto the precursor chabazitemolecular sieve. For example, a precursor chabazite molecular sieve isimmersed in a solution containing a soluble salt, e.g., copper nitrate,for a time sufficient to allow the incorporation of the copper cationsinto the precursor chabazite molecular sieve by ion exchange, and thenammonium hydroxide is added to incorporate the copper ions in thesolution onto the precursor chabazite molecular sieve by precipitation.The chabazite molecular sieve can then be washed, dried, and calcined.

In one embodiment, chabazite molecular sieve particles in the subjectinnovation can be finely divided particulate materials. The term “finelydivided” when utilized herein means that the particulate materials havea median individual particle size of about 10 microns or less. In oneembodiment, chabazite molecular sieve particles have a particle sizedistribution wherein at least about 90% by weight has a particle size ofabout 10 microns or less. In another embodiment, chabazite molecularsieve particles have a particle size distribution wherein at least about90% by weight has a particle size of about 3 microns or less. In yetanother embodiment, chabazite molecular sieve particles have a particlesize distribution wherein at least about 90% by weight has a particlesize of about 1 micron or less.

The fiber matrix wall flow filter can be coated with a chabazitemolecular sieve by any suitable technique. In one embodiment, the fibermatrix wall flow filter is coated with chabazite molecular sieve byimmersion techniques. The fiber matrix wall flow filter can be immersedvertically in a portion of a chabazite molecular sieve slurry. The topof the filter can be located just above the surface of the slurry. Inthis manner slurry contacts the inlet face of each honeycomb wall, butis prevented from contacting the outlet face of each wall. The filtercan be left in the slurry for about 30 seconds. The filter can beremoved from the slurry, and excess slurry is removed from the filterfirst by allowing it to drain from the channels, then by blowing withcompressed air (against the direction of slurry penetration), and thenby pulling a vacuum from the direction of slurry penetration. By usingthis technique, the catalyst slurry permeates the walls of the filter,yet the pores are not occluded to the extent that undue back pressurewill build up in the finished filter. As used herein, the term“permeate” when used to describe the dispersion of the catalyst slurryon the filter, means that the catalyst composition is dispersedthroughout the wall of the filter. The resulting fiber matrix wall flowfilter containing the chabazite molecular sieve SCR catalyst may bereferred to as a SCR filter.

The coated filters can be dried and then calcined. In one embodiment,the coated filter is dried at about 100 degrees Celsius and calcined ata higher temperature of about 300 degrees Celsius or more and about 450degrees Celsius or less. After calcining, the catalyst loading can bedetermined by calculation of the coated and uncoated weights of thefilter. Catalyst loading can be modified by altering the solids contentof the coating slurry. In one embodiment, repeated immersions of thefilter in the coating slurry can be conducted, followed by removal ofthe excess slurry as described above.

The coated filter can have any suitable concentration of chabazitemolecular sieve SCR catalyst compositions to ensure that the desiredNO_(x) reduction and particulate removal levels are achieved and/or tosecure adequate durability of the catalyst over extended use. In oneembodiment, SCR catalyst compositions are deposited on the filter at aconcentration of about 1 g/in³ or more and about 3 g/in³ or less. Inanother embodiment, SCR catalyst compositions are deposited on thefilter at a concentration of about 1.2 g/in³ or more and about 2.8 g/in³or less. In yet another embodiment, SCR catalyst compositions aredeposited on the filter at a concentration of about 1.4 g/in³ or moreand about 2.6 g/in³ or less. In still yet another embodiment, SCRcatalyst compositions are deposited on the filter at a concentration ofabout 1.5 g/in³ or more and about 2.5 g/in³ or less.

The claimed subject matter is described with reference to the drawings,wherein like reference numerals are used to refer to like elementsthroughout. In the following description, for purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the subject innovation. It may be evident, however,that the claimed subject matter may be practiced without these specificdetails. In other instances, well-known structures and devices are shownin block diagram form in order to facilitate describing the subjectinnovation.

FIGS. 1A and 1B illustrate a fiber matrix wall flow filter 100 which hasa plurality of passages 102. The passages 102 can be tubularly enclosedby the internal walls 104 of the filter substrate. The filter can havean inlet end 106 and an outlet end 108. Alternate passages can beplugged at the inlet end 106 with inlet plugs 110, and at the outlet end108 with outlet plugs 112 to form opposing checkerboard patterns at theinlet 106 and outlet 108. A gas stream 114 enters through the unpluggedchannel inlet 116, is stopped by outlet plug 112 and diffuses throughporous channel walls 104 to the outlet side 118. The exhaust gas cannotpass directly through the filter without crossing the walls because ofinlet plugs 110.

The fiber matrix wall flow filter can be catalyzed in that the wall offilter has thereon or contained therein one or more catalytic materials.Catalytic materials can be present on the inlet side of the element wallalone, the outlet side alone, or both the inlet and outlet sides. Thefiber matrix wall flow filter can contain one or more layers ofcatalytic materials and combinations of one or more layers of catalyticmaterials on the inlet and/or outlet walls of the element.

FIGS. 2A and 2B are schematic diagrams illustrating systems for treatingan exhaust stream containing NO_(x) and particulate matter.Specifically, FIG. 2A illustrates an exemplary emission treatment system200A for treating an exhaust stream containing NO_(x) and particulatematter using a SCR filter. In FIG. 2A, an exhaust 202 containing gaseouspollutants (including unburned hydrocarbons, carbon monoxide, andNO_(x)) and particulate matter is conveyed from an engine 204 to a SCRfilter 206. An oxidation catalyst (DOC) 208 can be optionally usedbetween the engine 204 and the SCR filter 206. Although not shown, thesystem 200A does not include an oxidation catalyst. In the oxidationcatalyst 208, unburned gaseous, non-volatile hydrocarbons (e.g.,volatile organic fraction (VOF)) and carbon monoxide can be combusted toform carbon dioxide and water. Removal of substantial proportions of theVOF using the oxidation catalyst can help to mitigate a deposition(e.g., clogging) of particulate matter on the SCR filter 206, which ispositioned downstream in the system. In one embodiment, a substantialportion of the NO of the NO_(x) in the exhaust is oxidized to NO₂ in theoxidation catalyst.

Ammonia or ammonia precursor (e.g., urea) can be injected as a spray viaa nozzle (not shown) into the exhaust stream. In one embodiment, aqueousurea shown on one line 210 serves as an ammonia precursor which can bemixed with air on another line 212 in a mixing device (MD) 214. A valve216 can be used to meter precise amounts of aqueous urea which areconverted in the exhaust stream to ammonia. The exhaust stream with theadded ammonia or ammonia precursor is conveyed to the SCR filter 206. Inone embodiment, on passing through the SCR filter, the NO_(x) componentis converted to NO by the particulate matter (e.g., soot cake) trappedon the SCR filter, and then NO is converted through the selectivecatalytic reduction of NO with ammonia to nitrogen.

The particulate matter including the soot fraction and the VOF can bealso largely removed by the SCR filter 206. In one embodiment, about 80wt. % or more of the particulate matter is removed by the SCR filter. Inanother embodiment, about 85 wt. % or more of the particulate matter isremoved by the SCR filter. In yet another embodiment, about 90 wt. % ormore of the particulate matter is removed by the SCR filter. Theparticulate matter deposited on the SCR filter 206 can be combustedthrough regeneration of the filter.

FIG. 2B illustrates another exemplary emission treatment system 200B fortreating an exhaust stream containing NO_(x) and particulate matterusing a SCR filter. In FIG. 2B, the emission treatment system 200Bincludes a slip oxidation catalyst 218 downstream of a SCR filter 206.The slip oxidation catalyst 218 can contain a composition containingbase metals and less than about 0.5 wt % of platinum. The slip oxidationcatalyst can be used to oxidize any excess NH₃ before it is vented tothe atmosphere. An oxidation catalyst 208 can be optionally used betweenthe engine and the SCR filter.

FIG. 3 illustrates an exemplary methodology 300 for reducing NO_(x)concentration and particulate matter in a diesel engine exhaust. At 302,ammonia or an ammonia precursor is injected into a diesel engineexhaust. At 304, the exhaust is passed through a SCR filter containing afiber matrix wall flow filter and a chabazite molecular sieve SCRcatalyst on the fiber matrix wall flow filter, the fiber matrix wallflow filter containing a plurality of non-woven inorganic fibers. In oneembodiment, the method further involves passing the exhaust through anoxidation catalyst before injecting ammonia or ammonia precursor intothe exhaust.

Although not shown in FIG. 3, in one embodiment, the fiber matrix wallflow filter contains at least one of alumina fibers, silica fibers,mullite fibers, silicon carbide fibers, aluminosilicate fibers, aluminumborosilicate fibers, or combinations thereof. The fiber matrix wall flowfilter can have a coefficient of thermal expansion of about1×10⁻⁶/degree Celsius or more and about 6×10⁻⁶/degree Celsius or less.The fiber matrix wall flow filter can have a modulus of rupture strengthof about 1,000 psi or more and about 2,000 psi or less. In anotherembodiment, the chabazite molecular sieve SCR catalyst compriseshydrophobic chabazite molecular sieve. The chabazite molecular sieve SCRcatalyst can contain metal exchanged chabazite molecular sieve. Forexample, the chabazite molecular sieve SCR catalyst contains at leastone of Cu-exchanged chabazite molecular sieve, Fe-exchanged chabazitemolecular sieve, or a combination thereof.

The following examples illustrate the subject innovation. Unlessotherwise indicated in the following examples and elsewhere in thespecification and claims, all parts and percentages are by weight, alltemperatures are in degrees Celsius, and pressure is at or nearatmospheric pressure.

Example 1

Example 1 shows evaluation of a back pressure for coated fiber matrixwall flow filters. A fiber matrix wall flow filter having dimensions of1×3 inches, an average pore size of 15 microns, and 67% wall porosity isused to prepare a catalyst-coated filter. A catalyst slurry is formedfrom copper-exchanged 3% chabazite molecular sieve (containing 3 wt. %of copper based on the weight of the chabazite molecular sieve) andde-ionized water.

The catalyst is deposited on the fiber matrix wall flow filter by (1)dipping the filter into the slurry to a depth sufficient to coat thechannels of the filter along the entire axial length of the filter fromone direction; (2) air-knifing the filter from the side opposite thecoating direction (e.g., the dry side); (3) vacuuming the filter fromthe coated side; and (4) drying the filter at about 93 degrees Celsiusfor about 1 hour in flowing air, and calcining the filter at about 400degrees Celsius for about 1 hour. Actions (1) through (4) are thenrepeated from the opposite side. The resulting fiber matrix wall flowfilter is designated as FMWFF. Pressure drop across of the resultingfiber matrix wall flow filters having three different catalyst loadingsis shown in Table 1.

Two cordierite ceramic wall flow filters are prepared in the similarmanner as comparative examples. The first cordierite ceramic wall flowfilter has dimensions of 1×6 inches, an average pore size of 18 microns,and 59% wall porosity. The second cordierite ceramic wall flow filterhas dimensions of 1×6 inches, an average pore size of 22 microns, and65% wall porosity. The first and second cordierite ceramic wall flowfilters are designated as CCWFF1 and CCWFF2, respectively. Pressure dropacross the cordierite ceramic wall flow filters is shown in Table 1.

TABLE 1 Increase in Pressure Filter Porosity drop after coating (%)FMWFF 67 12-16 CCWFF1 59 36-45 CCWFF2 65 22The pressure drop increase exhibited by FMWFF is lower than the pressuredrop of CCWFF1 and CCWFF2.

Example 2

Example 2 shows evaluation of NO_(x) conversion and NH₃ conversion by acoated fiber matrix wall flow filter and as a comparative example, acordierite ceramic wall flow filter (e.g., a SCR filter). FMWFF andCCWFF2 of Example 1 are used Both of the wall flow filter parts havedimensions of 1×6 inches The SCR catalyst is deposited on both of thecordierite ceramic wall flow filter with a catalyst loading of 2 g/in³in the manner as described in Example 1. Nitrogen oxides selectivecatalytic reduction (SCR) efficiency and selectivity of the freshcatalyst cores is measured by adding a feed gas mixture of 500 ppm ofNO, 500 ppm of NH₃, 10% O₂, 5% H₂O, balanced with N₂ to a steady statereactor containing a 1″D×3″L catalyst core. The reaction is carried at aspace velocity of 40,000 hr⁻¹ across a 150° C. to 460° C. temperaturerange. The resulting NOx conversions are presented in Table 2.

TABLE 2 NO NO2 NOx NH3 N2O make Outlet Temp % conversion % conversion %conversion % conversion ppm degrees C. CCWFF2 17.5 55.1 19.3 31.4 0.25156 49.9 76.6 51.2 53.1 1.06 206 89.1 95.7 89.4 90.4 2.34 255 97.6 98.697.7 96.7 2.18 309 98.9 98.6 98.9 97.8 2.39 351 FMWFF1 11.4 41.8 12.718.2 0.11 154 34.1 61.2 35.2 39.0 0.44 206 79.7 88.7 80.0 79.0 1.39 25097.0 93.1 96.8 96.0 1.24 305 98.8 94.3 98.6 97.1 1.08 349Thus, it can be seen that there is a general equivalence of thecatalytic activity between the two samples.

What has been described above includes examples of the disclosedinformation. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the disclosed information, but one of ordinary skill in theart can recognize that many further combinations and permutations of thedisclosed information are possible. Accordingly, the disclosedinformation is intended to embrace all such alterations, modificationsand variations that fall within the spirit and scope of the appendedclaims. Furthermore, to the extent that the term “includes,” “has,”“involve,” or variants thereof is used in either the detaileddescription or the claims, such term is intended to be inclusive in amanner similar to the term “comprising” as “comprising” is interpretedwhen employed as a transitional word in a claim.

1. A method of reducing NO_(x) and particulate matter in a diesel engineexhaust, comprising: injecting ammonia or an ammonia precursor into thediesel engine exhaust; and passing the exhaust through a SCR filtercomprising a fiber matrix wall flow filter and a chabazite molecularsieve SCR catalyst on the fiber matrix wall flow filter, the fibermatrix wall flow filter comprising a plurality of non-woven inorganicfibers.
 2. The method of claim 1 further comprising passing the exhaustthrough an oxidation catalyst before injecting ammonia or ammoniaprecursor into the exhaust.
 3. The method of claim 1, wherein the fibermatrix wall flow filter comprises at least one of alumina fibers, silicafibers, mullite fibers, silicon carbide fibers, aluminosilicate fibers,aluminum borosilicate fibers, or combinations thereof.
 4. The method ofclaim 1, wherein the fiber matrix wall flow filter has a coefficient ofthermal expansion from 200 to 800 ° C. of about 1×10⁻⁶/degree Celsius ormore and about 6×10⁻⁶/degree Celsius or less.
 5. The method of claim 1,wherein the fiber matrix wall flow filter has a porosity of about 50% ormore and about 70% or less.
 6. The method of claim 1, wherein the fibermatrix wall flow filter has a honeycomb structure.
 7. The method ofclaim 1, wherein the chabazite molecular sieve SCR catalyst comprisesmetal exchanged chabazite molecular sieve.
 8. The method of claim 7,wherein the chabazite molecular sieve SCR catalyst comprisesCu-exchanged chabazite molecular sieve, Fe-exchanged chabazite molecularsieve or a combination thereof.
 9. The method of claim 7, wherein thechabazite molecular sieve SCR catalyst has a surface that ishydrophobic.
 10. The method of claim 1, wherein the ammonia precursor isurea.